Technical Field
The present disclosure relates to semiconductor nanoparticles, a method of producing semiconductor nanoparticles, and a light-emitting device and a liquid crystal display device using the semiconductor nanoparticles.
Description of Related Art
White light-emitting devices used as backlights of a liquid crystal display devices and the like, that utilize photoluminescence from quantum dots (also called “semiconductor quantum dots”) have been proposed. Fine particles of semiconductor with a particle size of 10 nm or less, for example, are known to exhibit a quantum size effect. Such nanoparticles are called the quantum dots. The quantum size effect is a phenomenon where a valence band and a conduction band, each of which is regarded as continuous in bulk particles, become discrete when the particle size is on the nanoscale, whereby a bandgap energy changes depending on their particle size.
The quantum dots absorb light and emit light corresponding to the bandgap energy. Thus, the quantum dots can be used as a wavelength conversion material in the light-emitting devices. For example, Japanese Unexamined Patent Application Publication No. 2012-212862 and Japanese Unexamined Patent Application Publication No. 2010-177656 have proposed the light-emitting devices using the quantum dots. More specifically, part of light emitted from a light-emitting diode (LED) chip is absorbed by the quantum dots, which emit light of another color. The light emitted from the quantum dots and the light from the LED chip not absorbed by the quantum dots are mixed to produce white light. These patent documents have proposed the use of quantum dots made of group II-VI compounds such as CdSe and CdTe, or group IV-VI compounds such as PbS and PbSe. WO 2014/129067 has proposed a wavelength conversion film utilizing core-shell structured semiconductor quantum dots that are designed not to contain Cd or Pb, by taking into consideration the toxicity of compounds containing these elements. The formation of such a core-shell structure is also mentioned in non-patent document of Chem, Commun. 2010, vol. 46, pp. 2082-2084.
One of the advantages of using the quantum dots in the light-emitting devices is that the light with a wavelength corresponding to a bandgap can have a peak with a relatively narrow full width at half maximum. However, among the quantum dots proposed as the wavelength conversion material, only quantum dots made of a binary semiconductor, typified by a group II-VI semiconductor such as CdSe, can be confirmed to emit the light with the wavelength corresponding to the bandgap, that is, to achieve band-edge emission. Meanwhile, ternary quantum dots, especially, group I-III-VI quantum dots have not been confirmed to exhibit the band-edge emission.
The light emitted from the group I-III-VI quantum dots is caused by the defect levels of the surface or inside of the particles, or by the donor-acceptor-pair recombination, and thus has a broad emission peak with a wide full width at half maximum and a long photoluminescence lifetime. Such light emission is not appropriate for light-emitting devices, particularly, one that is used in the liquid crystal display device. This is because the light-emitting device used in the liquid crystal display device is required to emit light with a narrow full width at half maximum that has a peak wavelength corresponding to each of three primary colors (i.e., RGB) in order to ensure the high color reproducibility. For this reason, practical use of the ternary quantum dots has not been prompted despite its less toxic composition.